In U.S. Pat. No. 4,201,052, there is disclosed a hydraulic circuit control system comprising an actuator having opposed openings adapted to alternately function as inlets and outlets for moving the element of the actuator in opposite directions and a variable displacement pump with loading sensing control for supplying fluid to said actuator. A pilot operated spool type meter-in valve is provided to which the fluid from the pump is supplied and a pilot controller alternately supplies fluid at pilot pressure to the meter-in valve for controlling the direction and displacement of movement of the meter-in valve and the direction and velocity of the actuator. A pair of lines extend from the meter-in valve to the respective openings of the actuator and a pilot operated meter-out valve is associated with each line to the actuator for controlling the flow out of the actuator when the line to the actuator does not have pressure fluid from the pump applied thereto.
In such a hydraulic circuit, pressure compensation, and resultant constant flow, is achieved by utilization of flow forces in conjunction with the spring rate which tend to center the spool of the meter-in valve. The amount of pressure compensation may allow variation in flow when the pressure drop varies from the normal load sensing point.
In most cases, this performance is acceptable and the operator would not notice the change in flow in operating the actuator. However, in some cases, particularly motor applications, greater accuracy may be needed.
Accordingly, among the objections of the present invention is to provide greater accuracy of pressure compensation at low cost.
In accordance with the invention, pressure of fluid in the line to actuator, which does not have pressure fluid from the pump, is applied to the meter-in valve to apply a centering force which aids the pressure compensating flow forces to keep the flow constant. More specifically, feedback pins are associated with the spool of the meter-in valve and pressure from the line to the actuator which does not have pump pressure applied thereto, is applied to one of the pins to apply a centering force on the spool of the meter-in valve which aids the pressure compensating flow forces to keep the flow constant.